Brain tumors were induced in two of four adult owl monkeys inoculated with JC virus, a human polyomavirus. Impression smears prepared from unfixed tumor tissue showed cells to contain polyomavirus T-antigen, but virion antigens were not detected. In vitro cultured tumor cells also contained T-antigen, but were negative for virion antigen. Infectious virus was not isolated from extracts of the tumor. An owl monkey brain culture from a 45-day-old fetus showed cytopathology 15 days after inoculation with JC virus. JC virus T- an virion antigens were detected in this culture by immunofluorescence. Simian hemorrhagic fever virus was isolated from a chronically infected patas monkey. Unlike the lytic infection produced by the prototype strain, this isolate produced a persistent infection in MA-104 cells without noticeable cytopathology.